05.03.2010

Unintended energy impact from bioethanol production

Quelle: European Commission

Increasing crop production to meet demand for biofuels potentially increases the use of nitrogen fertilisers, which can lead to nitrate pollution in surface and groundwater. A new study is the first to consider the unintended extra energy needed to treat water to remove nitrate contamination in drinking water supplies.

In the United States, the Renewable Fuel Standard¹ (RFS) stipulates the volume of renewable fuel required to be blended into petrol must be increased from 9 billion gallons in 2008 to 36 billion gallons in 2022; 15 billion gallons of this can come from traditional first generation biofuels, such as bioethanol, produced from maize. In the EU, the Directive² on the promotion of the use of biofuels for transport mandates the share of biofuels be increased from 2 per cent of total fuel supply in 2005 to 5.75 per cent of total supply in 2010, based on the energy content.

The expansion in maize production required to meet this future demand for biofuels could potentially increase the use of nitrogen fertilisers. This raises concerns for the quality of water, which is likely to be affected by additional agricultural run-off and leaching of nitrogen contaminants into surface waters and groundwater. This in turn has implications for the extra energy that would be required to treat water that exceeds acceptable contamination levels.

Focusing on the Midwest Corn Belt region in the U.S., where the majority of maize is grown, the study investigated the unintended impact of the extra energy needed to purify contaminated water in order to meet the maximum contaminant levels³ (MCL) (10 milligrams per litre of nitrate measured as nitrogen) for acceptable U.S. drinking water standards. The study considered the 'reverse osmosis' method to remove nitrate as this is currently the most economical, energy-efficient and feasible option available at the commercial scale.

Although the majority of the public surface and groundwater sources in the U.S. have nitrate concentrations below the MCL, between 1998 and 2003 about 634 billion gallons (2400 billion litres) of water distributed in public supply systems for all household uses did not meet drinking water standards. If water used exclusively for drinking and not other purposes is considered, this volume falls to 57 billion gallons (215 billion litres).

Results from the study suggest the amount of energy consumed by using reverse osmosis to treat 634 billion gallons of water exceeding MCL standards would be approximately 2471 million kWh per year. To treat 57 billion gallons would consume 221 million kWh; as opposed to the minimal 111 million kWh and 10 million kWh per year respectively needed to undertake standard purification processes for water that has nitrate concentrations below the MCL. For both household water and drinking water this represents a 22-fold increase in energy consumption in the nitrate-affected areas to mitigate contamination and meet the MCL requirement.

The study projects that increases in nitrate contamination in water may have an impact on energy consumed in the water treatment sector because of the convergence of several related trends: (1) increasing cornstarch-based ethanol production, (2) increasing nutrient loading in surface water and groundwater resources as a consequence of increased corn-based ethanol production, (3) additional drinking water sources that exceed the MCL for nitrate, and (4) potentially more stringent drinking water standards for nitrate.

1. See: www.epa.gov/oms/renewablefuels/index.htm
2. See: http://ec.europa.eu/energy/renewables/biofuels/biofuels_en.htm
3. See: www.epa.gov/safewater/contaminants/index.html

Contact: kmt@mail.utexas.edu